We are studying murine intracisternal A-particles as a model for the evolution and expression of integrated reiterated viral genes. Multiple individual A-particle genes have been isolated from mouse DNA using recombinant DNA techniques. These genes, which number 1000-2000 copies per cell, make up a family of homologous but non-identical provirus-like elements which are highly conserved in various strains and tissues of Mus musculus. In the past year we have shown that sequences homologous to mouse IAP genes are present in other rodent genera and are particularly conserved in the Syrian hamster. We have extended our study of the genetic relationship between IAP's and an infectious extracellular virus derived from an Asian mouse species. The long terminal repeat segments of an IAP gene has been fully sequenced. We have begun to study the spatial relationship of IAP sequences to other mouse genes. A nuclear antigen has been detected in IAP negative cells using an antiserum raised against purified IAP structural proteins.